Work-space pressure regulator

ABSTRACT

A device and method for equalizing the pressure between work-space and crankcase in a pressurized engine, such as a Stirling engine. The device consists of a two-way valve connected between the work-space and the crankcase. The valve is connected to the work-space with a passageway including a constriction to provide an mean pressure for monitoring purposes. The valve connects the work-space and crankcase allowing the pressure to equalize when the mean pressure of the work-space exceeds the crankcase pressure by a predetermined amount.

TECHNICAL FIELD

The present invention pertains to regulating the pressure in thework-space of a pressurized engine, such as a Stirling engine.

BACKGROUND OF THE INVENTION

Stirling cycle machines, including engines and refrigerators, have along technological heritage, described in detail in Walker, StirlingEngines, Oxford University Press (1980), and incorporated herein byreference. The principle underlying the Stirling cycle engine is themechanical realization of the Stirling thermodynamic cycle:isovolumetric heating of a gas within a cylinder, isothermal expansionof the gas (during which work is performed by driving a piston),isovolumetric cooling, and isothermal compression.

A Stirling cycle engine operates under pressurized conditions. Stirlingengines contain a high-pressure working fluid, preferably helium,nitrogen or a mixture of gases at 20 to 140 atmospheres pressure. AStirling engine may contain two separate volumes of gases, a working gasvolume containing the working fluid, called a work-space or workingspace, and a crankcase gas volume, the gas volumes separated by pistonseal rings. The crankcase encloses and shields the moving portions ofthe engine as well as maintains the pressurized conditions under whichthe Stirling engine operates (and as such acts as a cold-end pressurevessel). A pressurized crankcase removes the need for high pressuresliding seals to contain the work-space working fluid and halves theload on the drive component for a given peak-to-peak work-spacepressure, as the work-space pressure oscillates about the mean crankcasepressure. The power output of the engine is proportional to thepeak-to-peak work-space pressure while the load on the drive elements isproportional to the difference between the work-space and the crankcasepressures. FIG. 1 shows typical pressures in the gas volumes for such anengine.

The action of the piston rings can raise or lower the mean workingpressure above or below the crankcase pressure, substantially mitigatingthe above-mentioned advantages of a pressurized crankcase. For example,manufacturing marks, deviations and molding details of the rings canproduce preferential gas flow in one direction between the work-spaceand the crankcase. The resulting difference in pressure between thework-space and the crankcase can produce as much as double the load onengine, while peak-to-peak pressure and thus engine power increases onlyfractionally (see, e.g., FIG. 2). In summary, pumping up the workspacemean pressure significantly increases engine wear with only a smallattendant increase in power production.

SUMMARY OF THE INVENTION

In embodiments of the present invention, a device is provided thatreduces the mean pressure difference between a work-space and apressurized engine crankcase of an engine, such as a Stirling engine.The device includes a valve connecting the work-space and crankcase ofthe engine. The pressure difference between work-space and crankcase ismonitored. When the mean pressure of the work-space differs from thecrankcase pressure by a predetermined amount, the valve opens, allowingthe pressure difference between the two spaces to equalize. When thepressure difference between the spaces is reduced sufficiently, thevalve closes, isolating the work-space from the crankcase. This closuremaximizing power production, while minimizing wear on drive components.

In a specific embodiment of the invention, pressure at which the valveopens is determined by a preloaded spring. In a further specificembodiment of the invention, the mean pressure is monitored by includinga constriction in the passageway from the valve to the work-space sothat a mean work-space pressure is presented to a pressure monitoringdevice. In a further specific embodiment of the invention, the devicefurther includes a constriction in the passageway from the crankcase tothe pressure monitoring device such that the monitoring device ispresented with a mean crankcase pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood by reference to thefollowing description, taken with the accompanying drawings, in which:

FIG. 1 shows a graph of work-space and crank-case pressure for aStirling engine with a pressurized crankcase;

FIG. 2 shows a graph of pressure between a work-space and a crankcasefor a Stirling engine when the work-space is pumped-up;

FIG. 3 shows a side view in cross section of a sealed Stirling cycleengine;

FIG. 4 shows a pressure regulator for an engine according to anembodiment of the invention;

FIG. 5 shows a pressure regulator for an engine according to anotherembodiment of the invention;

FIG. 6 shows a pressure regulator for an engine according to a furtherembodiment of the invention; and

FIG. 7 shows the pressure difference that may develop across a valveaccording to the embodiment shown in FIG. 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In embodiments of the present invention, a device is provided thatreduces the pressure difference between a work-space and a pressurizedengine crankcase of an engine, such as a Stirling engine. Referring toFIG. 3, a sealed Stirling cycle engine 50 is shown in cross section.While this embodiment of the present invention will be described withreference to the Stirling engine shown in FIG. 3, it should beunderstood that other engines, coolers, and similar machines maylikewise benefit from embodiments of the present invention and suchcombinations are within the scope of the invention, as described in theappended claims. A sealed Stirling cycle engine operates underpressurized conditions. Stirling engine 50 contains a high-pressureworking fluid, preferably helium, nitrogen or a mixture of gases at 20to 140 atmospheres pressure. Typically, a crankcase 70 encloses andshields the moving portions of the engine as well as maintains thepressurized conditions under which the Stirling engine operates (andacts as a cold-end pressure vessel.) A heater head 52 serves as ahot-end pressure vessel.

Stirling engine 50 contains two separate volumes of gases, a working gasvolume 80 and a crankcase gas volume 78, that will be calledhereinafter, a “work-space” and a “crankcase,” respectively. Thesevolumes are separated by piston rings 68, among other components. In thework-space 80, a working gas is contained by a heater head 52, aregenerator 54, a cooler 56, a compression head 58, an expansion piston60, an expansion cylinder 62, a compression piston 64 and a compressioncylinder 66. The working gas is contained outboard of the piston sealrings 68. The crankcase 78 contains a separate volume of gas enclosed bythe cold-end pressure vessel 70, the expansion piston 60, and thecompression piston 64. The crankcase gas volume is contained inboard ofthe piston seal rings 68.

In the Stirling engine 50, the working gas is alternately compressed andallowed to expand by the compression piston 64 and the expansion piston60. The pressure of the working gas oscillates significantly over thestroke of the pistons. During operation, fluid may leak across thepiston seal rings 68 because the piston seal rings 68 do not make aperfect seal. This leakage results in some exchange of gas between thework-space and the crankcase. A work-space pressure regulator (“WSPR”)84 serves to restore the pressure balance between the work-space and thecrankcase. In embodiments of the invention, the WSPR is connected to thework-space by passageway 82, which may be a pipe or other equivalentconnection, and to the crankcase by another passageway 86. When thework-space mean pressure 80 differs sufficiently from the mean crankcasepressure, the WSPR connects the two volumes via vent, 88 until thedifferential between the mean pressures diminishes.

For example, an exemplary work-space pressure regulator is shown in FIG.4. Pipe or passageway 82 connects the pressure regulator 84 to thework-space 80. A restrictive orifice 92 damps the oscillating work-spacepressure applying the mean work-space pressure to one end of theshuttle, 100. The orifice 92 is sized to be significantly larger thanthe piston seal ring leak. As used in this specification including anyappended claims, the term “constriction” will be used to denote anarrowing in a pipe or passageway, including such a constriction at theend of a pipe or passageway or any place within the pipe or passageway.The other end of the shuttle 100 is exposed to the crankcase pressurevia a pipe 86, which pipe may include a restrictive orifice 93 or otherconstriction. Orifice 93 may be sized much smaller than orifice 92, inwhich case the combination of the shuttle 100 and the orifice 93 act todamp movement of the shuttle from work-space pressure swings appliedthrough orifice 92. In a specific embodiment of the invention, orifice92, from WSGR to work-space is approximately 0.031 inches in diameter,while orifice 93, from WSGR to the crankcase, is approximately 0.014inches in diameter. In other embodiments of the invention, theconstriction from shuttle to crankcase may be omitted. Note that thecrankcase pressure is approximately constant over the piston's cycle,while the work-space pressure swings significantly during the cycle. Twosprings 102, 104 keep the shuttle 100 centered, when the mean work-spaceand the crankcase pressures are equal.

When the mean work-space pressure is higher than the crankcase pressure,the higher pressure moves the shuttle 100 to the right, compressingspring 104. If the pressure difference is large enough to expose port 88the work-space and the crankcase become connected. Some of thework-space gas flows into the crankcase until the two mean pressures areequalized, which allows the shuttle 100 to return to the originalposition, closing the port 88. Note that orifice from the work-space tothe WSGR 92 may be sized to allow the pressure to equalize betweenwork-space and crankcase quickly when port 88 is exposed, while stillsmall enough to present a mean work-space pressure to the shuttle 100.

When the mean crankcase pressure is higher than the work-space pressure,the shuttle will move to the left, compressing spring 102. If thepressure difference is large enough, port 88 will be exposed to channel112, connecting space 94 with the crankcase 78. Some of the crankcasegas flows into the work-space until the two mean pressures areequalized, which allows the shuttle 100 to return to its centeredposition, closing port 88.

The shuttle isolates the work-space 80 from the crankcase 78 in itscentered position. The seal may be provided by two cup seals 122 locatedat the end of shuttle nearest the crankcase vent 86 or by equivalentseals as are known in the art. Two ring seals 120 center and guide theshuttle 88 in the WSPR body 114.

Another embodiment of the invention is shown in FIG. 5 and labeledgenerally 200. Work-space housing 205 and crankcase housing 210 arebolted together capturing piston 215, work-space spring 225, andcrankcase spring 230 in their bores. The interface of the two housingscreates cup seal gland 260 into which seats a bidirectional cup seal220, and an O-ring gland 265 into which seats an O-ring 270. The O-ringseals the interior of the housings from the crankcase pressure. Twoorifices 235 allow the pressures inside the two housings to remain equalto the mean crankcase pressure and the mean work-space pressure,respectively, without large pressure oscillations or large mass flowsinto/out of the housings. The piston is free to move axially within thehousings by sliding on its bearing surfaces 250.

When the two pressures are equal, the springs keep the piston centeredsuch that the cup seal seals against the piston's sealing surface 255,preventing any flow between the two housings. When the pressuredifferential between the two housings becomes great enough, the forceimbalance on the piston will cause the piston to move away from theregion of high pressure, compressing the spring on the low-pressure sideand relaxing the spring on the high-pressure side. Equilibrium isreached when the pressure force imbalance equals the spring forceimbalance. If the pressure differential is great enough, the piston willbe displaced enough that the cup seal 220 no longer contacts the sealingsurface and instead loses sealing force against the decreasing diameterof the piston. Once the seal is broken, gas can flow from thehigh-pressure side, through the vent hole 240 or vent slot 245, past thecup seal 220, and into the adjacent housing. Gas will continue to flowuntil the pressure has equalized enough for the springs to return thepiston to a position where the cup seal 220 seals against the sealingsurface 255.

Another embodiment of the invention is shown in FIG. 6 and will bereferred to as the Preloaded WSPR (300). This embodiment of theinvention uses preloaded springs 302, 304 connected to an inner piston340 and an outer piston 342 to control working gas flow into and out ofthe work-space 80. The springs are open-coil springs and, thus, gasflows freely through these springs. WSPR 300 communicates with thework-space 80 via an orifice 392. Likewise, the crankcase volume 78 isconnected to WSPR 300 via port 393. Work-space pressure oscillations aredamped out by the constriction of the orifice 392 together with theforce of the pre-loaded springs 302, 304 acting on the pistons 340, 342.Seals 370, 372 provide a compliant seat for pistons 340, 342. Theorifice 392 is sized to be significantly larger than the piston sealring leak. WSPR 300 may be mounted on the compression cylinder head ofthe engine 58 (see FIG. 3).

The Preloaded WSPR relieves a mean overpressure in the work-space in thefollowing manner. The oscillating work-space pressure, which ispartially damped by the orifice 392, is applied to the face 380 of theinner piston 340 and to the face of the outer piston 342 that areproximate to the work-space. If the net mean pressure on the pistons isenough to overcome the preload on spring 302, then the inner and outerpistons move to the left and open the valve at 382. The released gasflows past the open seal at 382 around the outside of the outer piston342, through spring 302 and into the crankcase via port 393. Once thedifference between the work-space and the crankcase pressures dropsbelow the preload on spring 302, the outer piston 342 moves back to theright and seals at 382. Seal 372 provides a compliant seat for piston342.

The Preloaded WSPR relieves excess crankcase pressure by a similarmethod. When the net pressure times the inner piston's 340 area isgreater than the preload on spring 304, the inner piston 340 moves tothe right and opens the valve at 370, which provides a compliant sealfor the inner piston 340. Gas from the crankcase flows between the outerand inner pistons and into the work-space via the orifice at 392reducing the pressure differential. Once the difference between thework-space and the crankcase pressures drops below the preload on spring304, the inner piston 340 moves back to the left and seals at 370.

In another preferred embodiment of the invention, the preloads insprings 302 and 304 may be preloaded to different force levels. Thedifferent forces applied by the springs would allow the workspacepressure to “pump-up” (i.e., increase) reaching a higher mean pressure,thereby allow the engine to produce higher mechanical power. Thisembodiment allows the design to add engine power without raising thecrankcase mean pressure. Thus the power can be increased withoutredesigning or perhaps requalifying the crankcase pressure vessel.

The functioning of the Preloaded WSPR can be understood by consideringthe pressures difference between the two orifices 392 and 393 in FIG. 6.As an example, consider the pressure across valve 310, as shown in FIG.7. (It should be noted that FIG. 7 is exemplary only and does notrepresent measured data on a WSPR.) The pressure difference between thetwo orifices can be better described as the pressure difference acrossregulator valve 310 where the regulator valve is composed of the twopistons 340, 342, the two springs 302, 304 and the two valve seats 370,372. FIG. 7 shows the pressure across valve 310 for two cases. In onecase, the preload on each spring 302, 304 is the same, and the workspacedoes not “pump-up,” as shown by graph 402. The workspace and crank caseremain at approximately the same mean pressure. In the second case, thepreload on spring 302 is greater than the preload on spring 304. Graph404 shows the pressure across the valves, when the workspace has a meanpressure that is 100 psi above the crankcase pressure. In the lattercase, the pressure difference may become large enough to overcome thepreload on valve 302, opening valve 310 and allowing gas to flow out ofthe workspace into the crankcase, reducing the pressure in theworkspace. The horizontal line in FIG. 7 shows the pressure at which thepreload on spring 304 is overcome. At that pressure, the WSPR opensallowing gas to pass between workspace and crankcase. The devices andmethods described herein may be used in combination with componentscomprising other engines besides the Stirling engine in terms of whichthe invention has been described. The described embodiments of theinvention are intended to be merely exemplary and numerous variationsand modifications will be apparent to those skilled in the art. All suchvariations and modifications are intended to be within the scope of thepresent invention as defined in the appended claims

1. In an engine of the type having a working space, characterized by amean pressure, and a sealed crankcase, characterized by a crankcasepressure, an improvement comprising a valve in fluid communication withboth the working space and the crankcase, the valve permitting fluidflow between the working space and the crankcase when an absolute valueof a difference between the mean working space pressure and thecrankcase pressure exceeds a specified value.
 2. A device according toclaim 1 wherein the engine is a Stirling cycle engine.
 3. A deviceaccording to claim 1 wherein the pressure difference is the differencebetween the mean working space pressure and a mean crankcase pressure.4. A device according to claim 1 wherein the valve connection to theworking space includes a constriction.
 5. A device according to claim 4wherein the valve connection to the crankcase includes a constriction.6. A device according to claim 5, wherein the constriction in the valveconnection to the crankcase is smaller than the constriction in thevalve connection to the working space.
 7. A device according to claim 1,wherein a pressure at which the valve opens is determined by a preloadedspring.
 8. A device according to claim 1, wherein the device includes apiston to damp pressure oscillations.
 9. In an engine of the type havinga working space, characterized by a mean pressure, and a sealedcrankcase, characterized by a crankcase pressure, an improvementcomprising: a valve in fluid communication with both the working spaceand the crankcase, the valve permitting fluid flow from the workingspace to the crankcase when the working space pressure exceeds thecrankcase pressure by a first specified value and permitting fluid flowfrom the crankcase to the working space when the crankcase pressureexceeds the working space pressure by a second specified value.
 10. Adevice according to claim 9 wherein the first specified value exceedsthe second specified value.
 11. A method for minimizing a pressuredifference between a working space and a sealed crankcase in an engine,the method comprising: a. monitoring a pressure difference between theworking space and the crankcase and; b. opening a valve in fluidcommunication with the working space and the crankcase when the absolutevalue of the pressure difference exceeds a specified value.
 12. A methodaccording to claim 11 wherein the engine is a Stirling cycle engine. 13.A method according to claim 11 wherein the pressure difference is thedifference between the mean working space pressure and the crankcasepressure.
 14. A method according to claim 11 wherein the pressuredifference is the difference between the mean working space pressure andthe mean crankcase pressure.
 15. A method according to claim 11 whereinthe valve connection to the working space includes a constriction.
 16. Amethod according to claim 11 wherein the valve connection to thecrankcase includes a constriction.
 17. A method according to claim 11,wherein the valve includes a piston to damp pressure oscillations.
 18. Amethod according to claim 11, wherein a pressure at which the valveopens is determined by a preloaded spring.
 19. A method for minimizing apressure difference between a working space and a sealed crankcase in anengine, the method comprising: a. monitoring a pressure differencebetween the working space and the crankcase and; b. opening a valve influid communication with the working space and the crankcase when theworking space pressure exceeds the crankcase pressure by a firstspecified value; and c. opening the valve in fluid communication withthe working space and the crankcase when the crankcase pressure exceedsthe working space pressure by a second specified value.
 20. A methodaccording to claim 19, wherein the first specified value exceeds thesecond specified value.